In magnetic resonance imaging, an object to be imaged as, for example, a body of a human subject, is exposed to a strong, substantially constant static magnetic field. The static magnetic field causes the spin vectors of certain atomic nuclei within the body to randomly rotate or “precess” around an axis parallel to the direction of the static magnetic field. Radio frequency excitation energy is applied to the body, and this energy causes the nuclei to precess in phase and in an excited state. As the precessing atomic nuclei relax, weak radio frequency signals are generated and emitted; such radio frequency signals are referred to herein as magnetic resonance signals.
Different tissues produce different signal characteristics. Tissue relaxation times are the dominant factor in determining signal characteristics. In addition, tissues having a high density of certain nuclei will produce stronger signals than tissues with a low density of such nuclei. Relatively small gradients in the magnetic field are superimposed on the static magnetic field at various times during the process so that magnetic resonance signals from different portions of the patient's body differ in phase, amplitude and/or frequency. If the process is repeated numerous times using different combinations of gradients, the signals from the various repetitions together provide enough information to form a map of signal characteristics versus location within the body. Such a map can be reconstructed by conventional techniques well known in the magnetic resonance imaging art, and can be displayed as a pictorial image of the tissues as known in the art.
The magnetic resonance imaging technique offers numerous advantages over other imaging techniques. MRI does not expose either the patient or medical personnel to X-rays and offers important safety advantages. Also, magnetic resonance imaging can obtain images of soft tissues and other features within the body which are not readily visualized using other imaging techniques. Accordingly, magnetic resonance imaging has been widely adopted in the medical and allied arts.
Many MRI systems use one or more solenoidal superconducting coils to provide the static magnetic field arranged so that the patient is disposed within a small tube running through the center of the coils. The coil and tube typically extend along a horizontal axis, so that the long axis or head-to-toe axis of the patient's body is in a horizontal position during the procedure.
Other MRI systems use iron core magnets to provide a more open environment for the patient. These magnets typically have a ferromagnetic frame with a pair of ferromagnetic poles disposed one over the other along a vertical pole axis with a gap between them for receiving the patient. The frame includes ferromagnetic flux return members such as plates or columns which are located outside the patient receiving area and extend vertically. A magnetic field is provided by permanent magnets or electromagnetic coils (superconductive or resistive) associated with the frame. A magnet of this type can be designed to provide a more open environment for the patient.
Ferromagnetic frame magnets having horizontal pole axes have also been developed. As disclosed, for example, in commonly assigned U.S. Pat. No. 6,414,490, issued on Jul. 2, 2002, and U.S. Pat. No. 6,677,753, issued on Jan. 13, 2004, the disclosures of which are incorporated by reference herein in their entirety, a magnet having poles spaced apart from one another along a horizontal axis provides a horizontally oriented magnetic field within a patient receiving gap between the poles. Such a magnet can be used with a patient positioning device including elevation and rotation mechanisms to provide extraordinary versatility in patient positioning. For example, where the patient positioning device includes a bed or similar device for supporting the patient recumbent position, the bed can be tilted and/or elevated so as to image the patient in essentially any position between a fully standing position and a fully recumbent position, and can be elevated so that essentially any portion of the patient's anatomy is disposed within the gap in an optimum position for imaging.
In each of the above described systems, the quality of an image generated from the (magnetic resonance) MR signals generated by the precessing nuclei will depend on the signal-to-noise ratio (SNR) between the MR signal and other noise emitted from or around the MR imaging system. As such, it is important to increase the SNR of the generated MR signals in order to achieve high quality MR imaging.